Alkali-soluble nitrone compounds and contrast enhanced material comprising the same

ABSTRACT

Novel nitrone compounds of the following formula are provided ##STR1## wherein R 1 , R 2  and R 3  may be the same or different and represent an alkyl group, an aryl group or a hydrogen atom, R 4  to R 8  may be the same or different and represent an alkyl group, a hydrogen atom or a carboxyl group provided that at least one of R 4  to R 8  is a carboxyl group, X represents an alkoxy group of the formula, R 9  O--, wherein R 9  represents an alkyl group, a dialkylamino group of the formula, R 10  R 11  N--, wherein R 10  and R 11  may be the same or different and represent an alkyl group, or a hydrogen atom, and n is a value of 0, 1 or 2. A contrast enhanced material comprising the nitrone compound is also provided, which is able to enhance a contrast when exposed to light having a wavelength of 300 to 450 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to novel nitrone compounds which are suitable foruse as a main ingredient of a contrast enhanced material for enhancingan image contrast of a subject, like a mask for photoetching used forfabrication, for example, of semiconductive integrated circuits and alsoto contrast enhanced materials which comprises the nitrone compounds.

2. Description of the Prior Art

In lithographic techniques, a resist image which is obtained by exposinga photo-resist to light in a desired pattern and developing the patternhas more vertically shaped walls when the contrast of the exposed imageis greater. When the exposure is carried out at a very high resolution,the contrast of the exposed image is lowered with the attendant problemthat a clear resist image cannot be obtained.

For increasing the resolution and obtaining a high resolution pattern,there have been proposed methods of forming resist patterns wherein acontrast enhanced layer for enhancing the contrast which has anabsorption maximum against light having a wavelength of 300 to 450 nm(Japanese Patent Publication No. 62-40697 and Japanese Laid-Open PatentApplication No. 62-234148). The nitrone compounds used as a mainingredient for the contrast enhanced material have the followingstructures. ##STR2##

However, the above-mentioned nitrone compounds are insoluble in waterand are soluble only in organic solvents. Accordingly, the contrastenhanced materials using the nitrone compounds cannot be soluble inwater. This requires the step of removing the contrast enhanced layerwith use of organic solvents prior to the developing step of aphotoresist. Alternatively, if an intervening layer is provided, it isessentially required to force the intervening layer and the contrastenhanced layer to be blown off by means of pure water. The formerremoving step using the organic solvent cannot be performed in existingapparatus, inviting complication of the fabrication procedure. With thelatter procedure wherein pure water is used to blow the layer off alongwith the intervening layer, there is the problem that the contrastenhanced layer cannot be completely blown off but is partly left, thusproducing scums. Accordingly, there is the demand for nitrone compoundswhich are useful as an ingredient for contrast enhanced materials andwhich do not invite complication of the steps and are soluble in purewater.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide novelalkali-soluble nitrone compounds which are soluble in pure water andwhich are useful as a main ingredient of contrast enhanced materials.

It is another object of the invention to provide a contrast enhancedmaterial which comprises the novel nitrone compound.

The above objects can be achieved, according to one embodiment of theinvention, by a novel alkali-soluble nitrone compound of the followingformula ##STR3## wherein R¹, R² and R³ may be the same or different andrepresent an alkyl group, an aryl group or a hydrogen atom, R⁴ to R⁸ maybe the same or different and represent an alkyl group, a hydrogen atomor a carboxyl group provided that at least one of R⁴ to R⁸ is a carboxylgroup, X represents an alkoxy group of the formula, R⁹ O--, wherein R⁹represents an alkyl group, a dialkylamino group of the formula, R¹⁰ R¹¹N--, wherein R¹⁰ and R¹¹ may be the same or different and represent analkyl group, or a hydrogen atom, and n is a value of 0, 1 or 2.

The novel nitrone compound can be obtained, for example, by subjectingan alkylnitrobenzoic acid of the following formula (2) to hydrogenation,followed by reaction with a compound of the general formula (3) toobtain novel, alkali-soluble nitrone compounds of the following formula(1) ##STR4## wherein R¹ to R⁸ have, respectively, the same meanings asdefined above.

The novel nitrone compounds have been found to be useful as an mainingredient for contrast enhanced materials and have shown great effectsin the contrast enhanced lithography using light having a wavelength offrom 300 to 450 nm.

More particularly, the novel nitrone compounds of the formula (1) isalkali-soluble and has contrast enhancing properties against lighthaving a wavelength of from 300 to 450 nm. Thus, it is possible toprovide a water-soluble contrast enhanced material which has never beenexpected using existing nitrone compounds. This means that a contrastenhanced layer may be removed not only with organic solvents, but alsowith water. Existing apparatus may be used as it is without causing thesteps to be complicated and without formation of any scum when using anintervening layer. Accordingly, when the nitrone compound of the formula(1) is used to prepare a contrast enhanced material for carrying out thecontrast enhanced lithography at g line (light with a wavelength of 436nm) of an i line (light with a wavelength of 365 nm), there can beformed a fine resist pattern which has high contrast, high resolutionand high precision.

According to another embodiment of the invention, there is also provideda contrast enhanced material which comprises a nitrone compound of theformula (1).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(d) are a flow sheet showing a series of lithographicsteps using a contrast enhanced material according to one embodiment ofthe invention; and

FIGS. 2(a) to 2(e) are a flow sheet showing a series of lithographicsteps using a contrast enhanced material according to another embodimentof the invention.

DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION

As stated before, the novel nitrone compound of the invention has thefollowing general formula (1). ##STR5## In the above formula, R¹, R² andR³ are independently an alkyl group, an aryl group or a hydrogen atom.Examples of the alkyl group are those having from 1 to 8 carbon atoms,including a methyl group, an ethyl group, a propyl group, an iso-propylgroup, a butyl group, a sec-butyl group, a tert-butyl group, acyclohexyl group and the like. Of these, there are preferred a methylgroup, an ethyl group, a propyl group and a butyl group. The aryl groupsare those having from 6 to 15 carbon atoms and include, for example, aphenyl group, substituted phenyl groups, a naphthyl group, substitutednaphthyl groups and the like. Of these, a phenyl group, a methylphenylgroup, and an ethylphenyl group are preferred. R⁴, R⁵, R⁶, R⁷ and R⁸ maybe the same or different and represent an alkyl group or a hydrogen atomor a carboxyl group provided that at least one of R⁴, R⁵, R⁶, R⁷ and R⁸a carboxyl group. The alkyl group in this case may be those indicatedwith respect to R¹, R² and R³. X represents an alkoxy group of theformula, R⁹ O--, wherein R⁹ represents an alkyl group, a dialkylaminogroup of the formula, R¹⁰ R¹¹ N--, wherein R¹⁰ and R¹¹ may be the sameor different and represent an alkyl group, or a hydrogen atom. The alkylgroup for R⁹, R¹⁰ and R¹¹ is one represented by R¹¹, R² and R³. Letter nis a value of 0, 1 or 2, preferably 0 or 1.

More specific examples of the nitrone compound of the formula (1)include those of the following formulae. ##STR6## (wherein m=1 or 2)

Specific examples of the nitrone compounds of the formula (1) includeα-[p-(dimethylamino)phenyl]-N-(4-carboxyphenyl)nitrone,α-[p-(diethylamino)phenyl]-N-(4-carboxyphenyl)nitrone,α-[p-(dibutylamino)phenyl]-N-(4-carboxyphenyl)nitrone,α-[p-(diethylamino)phenyl]-N-(3-carboxyphenyl)nitrone,α-[p-(diethylamino)phenyl]-N-(2-carboxyphenyl)nitrone,α-[p-(diethylamino)phenyl]-N-(2-methyl -4-carboxyphenyl)nitrone,α-[p-(diethylamino)phenyl]-N-(2-methyl-3-carboxyphenyl)nitrone,α-(p-methoxyphenyl)-N-(4-carboxyphenyl)nitrone,α-(p-ethoxyphenyl)-N-(4-carboxyphenyl)nitrone,α-(o-methoxyphenyl)-N-(4-carboxyphenyl)nitrone,α-(p-methoxyphenyl)-N-(3-carboxyphenyl)nitrone,α-(p-methoxyphenyl)-N-(2-carboxyphenyl)nitrone,α-(p-methoxyphenyl)-N-(2-methyl-4-carboxyphenyl)nitrone,α-(p-ethoxyphenyl)-N-(3-methyl-2carboxy-phenyl)nitrone,α-[p-(dimethylamino)styryl]-N-(4-carboxypenyl)nitrone,α-[p-(diethylamino)styryl]-N-(4-carboxyphenyl)nitrone,α-[p-(dimethylamino)styryl]-N-(3-carboxyphenyl)nitrone,α-[p-(diethylamino)styryl]-N-(2-methyl -4-carboxypheyl)nitrone,α-[p-(diethylamino)styryl]-N-(4-methyl-2-carboxyphenyl)nitrone,α-(o-methoxystyryl)-N-(4-carboxyphenyl)nitrone,α-(o-methoxystyryl)-N-(3-carboxyphenyl)nitrone,α-(p-methoxystyryl)-N-(4-carboxyphenyl)nitrone,α-(p-methoxystyryl)-N-(3-carboxyphenyl)nitrone,α-(o-methoxystyryl)-N-(2-methyl-4-carboxyphenyl)nitrone,α-(p-methoxystyryl)-N-(2-methyl-3-carboxyphenyl)nitrone and the like.

The nitrone compounds of the formula (1) of the invention can beprepared by providing, as a starting material, an alkylnitrobenzoic acidof the following formula (2) which are readily available on anindustrial scale, subjecting the alkylnitrobenzoic acid of the formula(2) to hydrogenation reaction according to the following reactionsequence to obtain a compound of the following general formula (4), andsubjecting to further reaction with a compound of the general compound(3). ##STR7##

The above reaction should preferably be carried out in organic solventssuch as methanol. The hydrogenation of the alkylnitrobenzoic acid of theformula (2) is performed at room temperature in the presence of acatalytic amount of a catalyst such as a platinum-on-carbon catalyst insuch a way that 1.7 to 2.3 equivalents of hydrogen relative to thealkylnitrobenzoic acid are filled at a pressure of from 1 to 10 kg/cm².

For the reaction between the hydrogenated alkylnitrobenzoic acid and thecompound of the formula (3), it is preferred that the compound of theformula (3) is added in amounts of from 0.8 to 1.2 moles per mole of thealkylnitrobenzoic acid and the reaction is effected in the presence ofan acid such as acetic acid. The reaction conditions should preferablyinclude a temperature of from 20° to 50° C. and a time of from 3 to 8hours. After completion of the reaction,, the precipitated crystals areremoved by filtration and washed with appropriate solvents to obtain anintended nitrone compound of the formula (1).

The contrast enhanced material of the invention should comprise thenitrone compound of the formula (1). The nitrone compound may bedissolved in water or organic solvents along with binders, organicbases, surface active agents and the like to provide a rotationalcasting contrast enhanced material. The contrast enhanced materialshould preferably comprise (A) from 0 to 100 parts by weight(hereinafter referred to simply as parts), preferably from 50 to 100parts, of water, (B) from 0 to 100 parts, preferably from 0 to 50 parts,of an organic solvent, (C) from 1 to 30 parts, preferably from 1 to 15parts, of a water-soluble polymer binder, (D) from 1 to 30 parts,preferably from 1 to 15 parts, of the nitrone compound, (E) from 1 to 30parts, preferably from 1 to 15 parts, of an organic base, and (F) from 0to 2 parts, preferably from 0 to 1 part, of a surface active agent. Aswill be apparent from the above, the contrast enhanced material shouldessentially contain the ingredients (C), (D) and (E).

The organic solvents used as the (B) ingredient include, for example,alcohols such as ethanol, 1-propanol, 2-propanol and the like, andethers such as tetrahydrofuran, 1,4-dioxane and the like.

The binders used as the (C) ingredient include, for example, partiallysaponified products of vinyl acetate polymers, polyvinyl alcohol,water-soluble cellulose ethers or cellulose esters, vinyl pyrrolidonehomopolymers or copolymers, pullulans and the like.

The organic bases used as the (E) ingredient include, for example,tetramethylammonium hydroxide, tetrabutylammonium hydroxide,tris(hydroxymethyl)aminomethane, 2,2',2"-nitrilotriethanol, pyridine,triethylamine and the like.

The surface active agents used as the (F) ingredient include, forexample, fluorine-containing surface active agents.

For the formation of a resist pattern from the contrast enhancedmaterial according to the invention, lithographic procedures as shown inFIGS. 1 and 2 are used. FIG. 1 shows an embodiment wherein a contrastenhanced layer is directly formed on a photoresist layer. Moreparticularly, a photoresist layer 2 is formed on a substrate 1, such asa silicon wafer, by a coating technique such as spin coating.Subsequently, a contrast enhanced material of the invention is applied,such as by spin coating, onto the photoresist layer 2 to form a contrastenhanced layer 3. The contrast enhanced layer 3 is exposed to a UV raywith a predetermined wavelength in an imagewise pattern according to areduced projection method. In FIG. 1, portions A are exposed to the ray,after which the contrast enhanced layer 3 is removed by means of water,followed by developer to form a resist pattern 5.

In FIG. 2, there is shown an embodiment wherein a thin intervening layer6 made of a neutral material such as polyvinyl alcohol is providedbetween the contrast enhanced layer and the photoresist layer so thatboth layers are kept away from each other. In this case, since thecontrast enhanced material of the invention is soluble in water, thecontrast enhanced layer 3 and the intervening layer 6 are both washedaway with water, so that any scum is not produced.

The nitrone compound of the formula (1) of the invention sufferscontrast enhancement against g and i lines and is soluble in alkalis orbases, with which there can be provided water-soluble contrast enhancedmaterials which are capable of forming a fine resist pattern with highcontrast, high resolution and high precision. Thus, the nitrone compoundis very useful as a main ingredient of a water-soluble contrast enhancedmaterial for contrast improvement by which a fine resist pattern can beformed.

The invention is more particularly described by way of examples, whichshould not be construed as limiting the invention.

EXAMPLE 1

122.2 g (731 mmols) of p-nitrobenzoic acid, 11.3 g of dimethylsulfoxide,2.0 g of a 5% platinum-supported catalyst, and 260 g of methanol werecharged into a one liter autoclave, followed by filling hydrogen at apressure of about 4 kg/cm² and continuing agitation at room temperatureuntil an equivalent of hydrogen was reacted with p-nitrobenzoic acid.After removal of the catalyst, 120.0 g (677 mmols) of4-diethylaminobenzaldehyde, 30.0 g of acetic acid and 100 g of methanolwere added to the resultant residue, followed by agitation at roomtemperature for 5 hours. After removal of the resultant crystals byfiltration, they were washed with methanol to obtain yellow crystals anddried to isolate 187.6 g (yield 88.7%) ofα-p-(diethylamino)phenyl]-N-(4-carboxyphenyl)nitrone with a purity of96%. The thus obtainedα-[p-(diethylamino)-phenyl]-N-(4-carboxyphenyl)nitrone had the followingresults with respect to mass spectra (MS), NMR spectra (NMR), IR spectra(IR) and elementary analysis.

MS: m/e spectral intensity ratio

312 (16.5), 297 (8.1), 296 (15.1), 281 (24.6), 176 (100.0), 32 (31.5)

¹ H-NMR: δ(ppm)

    ______________________________________                                         ##STR8##                                                                 

    ______________________________________                                        (a)      1.2            triplet  6H                                           (b), (h) 3.3-3.8        multiplet                                                                              5H                                           (c)      8.4            doublet  2H                                           (d)      6.8            doublet  2H                                           (e)      8.3            singlet  1H                                           (f), (g) 8.8            singlet  4H                                           ______________________________________                                    

IR: (cm⁻¹)

3437, 2976, 2900, 1705, 1605, 1572, 1523, 1410, 1275, 1180, 1155, 1122,1080, 1036, 1011, 879, 860, 817

Elementary analysis: (%) C₁₈ H₂₀ N₂ O₃ Calculated C: 69.2; H: 6.4.; N:9.0; Found C: 68.1; H: 6.2; N: 8.5.

EXAMPLE 2

The general procedure of Example 1 was repeated except that 132.4 g (731mmols) of 3-methyl-4-nitrobenzoic acid was used instead ofp-nitrobenzoic acid, thereby obtaining 211.3 g (yield 95.6%) ofα-[p-(diethylamino)phenyl]-N-(2-methyl-4-carboxyphenyl)nitrone with apurity of 95%. The results of the analyses of the thus obtained compoundare shown below.

MS: m/e spectral intensity ratio

326 (20.3), 311 (15.3), 310 (6.7), 295 (8.9), 176 (100.0), 32 (26.0)

¹ H-NMR: δ(ppm)

    ______________________________________                                         ##STR9##                                                                 

    ______________________________________                                        (a)    1.1            triplet   6H                                            (b)    3.4            quadruplet                                                                              4H                                            (c)    6.6-8.3        multiplet 8H                                            (d)    2.4            singlet   3H                                            (e)    3.2            singlet   1H                                            ______________________________________                                    

IR: (cm⁻¹)

3435, 3053, 2974, 2902, 1703, 1591, 1524, 1408, 1273, 1176, 1155, 1122,1078, 1039, 1009, 877, 854, 817

Elementary analysis: (%) C₁₉ H₂₂ N₂ O₃ ; Calculated C: 69.9; H: 6.7; N:8.6; Found C: 67.4; H: 6.4; N: 8.0.

EXAMPLE 3

The general procedure of Example 1 was repeated except that 92.2 g (677mmols) of p-anisaldehyde was used instead of 4-diethylaminobenzaldehyde,thereby obtaining 162.0 g (yield 88.2%) of α-(p-methoxyphenyl)-N-(4-carboxyphenyl)nitrone with a purity of 96%. The results of the analysesof the thus obtained compound are shown below.

¹ H-NMR: δ(ppm)

    ______________________________________                                         ##STR10##                                                                

    ______________________________________                                        (a)    3.84              singlet 3H                                           (b)    8.51              doublet 2H                                           (c)    7.07              doublet 2H                                           (d)    8.49              singlet 1H                                           (e)    8.07              doublet 2H                                           (f)    8.08              doublet 2H                                           (g)    not detected              1H                                           ______________________________________                                    

IR: (cm⁻¹)

3456, 3109, 3157, 2968, 2841, 2673, 2551, 1703, 1686, 1603, 1508, 1431,1323, 1292, 1251, 1169, 1070, 1028, 870, 841, 806

Elementary analysis: (%) C₁₅ H₁₃ NO₄ ; Calculated C: 66.4; H: 4.8; N:5.2; Found C: 66.1; H: 4.9; N: 4.8.

EXAMPLE 4

The general procedure of Example 1 was repeated except that 132.4 g (731mmols) of 3 -methyl-4-nitrobenzoic acid was used instead ofp-nitrobenzoic acid and 118.6 g (677 mmols) of4-dimethylaminocinnamaldehyde was used instead of4-diethylaminobenzaldehyde, thereby obtaining 213.8 g (yield 97.4%) ofα-[p-(dimethylamino)styryl]-N-(2-methyl-4-carboxyphenyl)nitrone with apurity of 96%. The results of the analyses of the thus obtained compoundare shown below.

¹ H-NMR: δ(ppm)

    ______________________________________                                         ##STR11##                                                                

    ______________________________________                                        (a)    3.00              singlet 6H                                           (b)    6.75              doublet 2H                                           (c)    7.47              doublet 2H                                           (d)    7.26              singlet 1H                                           (e)    7.27              doublet 1H                                           (f)    7.78              doublet 1H                                           (g)    7.89              doublet 1H                                           (h)    7.50              doublet 1H                                           (i)    7.94              singlet 1H                                           (j)    2.40              singlet 3H                                           (k)    not detected              1H                                           ______________________________________                                    

IR: (cm⁻¹)

3446, 3037, 2891, 2812, 1701, 1593, 1524, 1487, 1435, 1367, 1329, 1275,1234, 1232, 1165, 1126, 1105, 1018, 984, 945, 924, 920, 860, 841, 810

Elementary analysis: (%) C₁₉ H₂₀ N₃ O₂ ; Calculated C: 70.4; H: 6.2; N:8.6; Found C: 69.9; H: 6.3; N: 8.4.

EXAMPLE 5

As a contrast enhanced material, there was provided an aqueous solutionwhich contained 4.5% ofα-[p-(diethylaminophenyl]-N-(4-carboxyphenyl)nitrone, 3.5% of a vinylpyrrolidone-vinyl acetate copolymer having a molecular weight of about3,000 and a monomer ratio of 6:4, 38.0% of a 10% tetra-n-butylammoniumhydroxide solution, and 0.25% of Florad FC-430 (available from Sumitomo3M Co., Ltd.) to form a resist pattern according to the lithographicprocess shown in FIG. 1. More particularly, S-1813 (positive resist,available from Shipray Co., Ltd.) was spin coated on a substrate 1 madeof a silicon wafer to form a resist layer 2 (FIG. 1(a)). Then, thecontrast enhanced material indicated above was spin coated onto theresist layer 2 to form a contrast enhanced layer 3 (FIG. 1(b)).Thereafter, a UV ray 4 having a wavelength of 436 nm was selectivelyexposed on portions A according to a reduced projection method (FIG.1(c)). Finally, the contrast enhanced layer 3 was removed with use ofpure water, followed by development with an alkaline developer to form aresist pattern 5. (FIG. 1(d)). The thus obtained resist pattern wasenhanced in contrast with a resolution of 0.5 μm.

EXAMPLE 6

There was provided, as a contrast enhanced material, an aqueous solutionwhich contained 4.5 % ofα-[p-(diethylaminophenyl-N-(2-methyl-4-carboxyphenyl)nitrone, 3.5% of avinyl pyrrolidone-vinyl acetate copolymer having a molecular weight ofabout 3000 and a monomer ratio of 6:4, 2.0% oftris(hydroxymethyl)aminomethane, and 0.25% of Florad FC-430 (availablefrom Sumitomo 3M Co., Ltd.) to form a resist pattern according to thelithographic process shown in FIG. 1. More particularly, THMR-iP1800(positive resist, available from Tokyo Applied Chem. Ind. Co., Ltd.) wasspin coated on a substrate 1 made of a silicon wafer to form a resistlayer 2 (FIG. 1(a)). Then, the contrast enhanced material indicatedabove was spin coated onto the resist layer 2 to form a contrastenhanced layer 3 (FIG. 1(b)). Thereafter, a UV ray 4 having a wavelengthof 365 nm was selectively exposed on portions A according to a reducedprojection method (FIG. 1(c)). Finally, the contrast enhanced layer 3was removed with use of pure water, followed by development with analkaline developer to form a resist pattern 5 (FIG. 1(d)). The thusobtained resist pattern was enhanced in contrast with a resolution of0.36 μm.

EXAMPLE 7

The general procedure of Example 6 was repeated except that there wasused α-(p-methoxyphenyl)-N-(4-carboxyphenyl)nitrone instead ofα-[p-(diethylaminophenyl)-N-(2-methyl-4-carboxyphenyl)nitrone, therebyforming a resist pattern whose contrast was enhanced with a resolutionof 0.4 μm.

EXAMPLE 8

The general procedure of Example 5 was repeated except that there wasused a pure water-1-propanol solution (ratio by weight of 65:35) whichcontained 4.5% ofα-[p-dimethylamino)styryl]-N-(2-methyl-4-carboxyphenyl)nitrone, 3.5% ofvinyl pyrrolidone-vinyl acetate copolymer having a molecular weight ofabout 3,000 and a monomer ratio by weight of 6:4, 2% oftris(hydroxymethyl)aminomethane, and 0.25% of Floprad FC-430 (availablefrom Sumitomo 3M Co., Ltd.), thereby forming a resist pattern. Thepattern was enhanced in contrast with a resolution of 0.5 μm.

What is claimed is:
 1. An alkali-soluble nitrone compound of thefollowing formula ##STR12## wherein R¹, R² and R³ may be the same ordifferent and represent an alkyl group, an aryl group or a hydrogenatom, R⁴ to R⁸ may be the same or different and represent an alkylgroup, a hydrogen atom or a carboxyl group provided that at least one ofR⁴ to R⁸ is a carboxyl group, X represents an alkoxy group of theformula, R⁹ O--, wherein R⁹ represents an alkyl group, a dialkylaminogroup of the formula, R¹⁰ R¹¹ N--, wherein R¹⁰ and R¹¹ may be the sameor different and represent an alkyl group, or a hydrogen atom, and n isa value of 0, 1 or
 2. 2. An alkali-soluble nitrone compound according toclaim 1, wherein R¹, R² and R³ are independently an alkyl group havingfrom 1 to 8 carbon atoms.
 3. An alkali-soluble nitrone compoundaccording to claim 1, wherein R¹, R² and R³ are independently an arylgroup having from 6 to 15 carbon atoms.
 4. An alkali-soluble nitronecompound according to claim 1, wherein n is 0 or
 1. 5. An alkali-solublenitrone compound according to claim 1, wherein said nitrone compound isof the formula ##STR13## wherein R³, R⁴, R⁵, R⁶, R⁷, R⁸, R¹⁰ and R¹¹have, respectively, the same meanings as defined in claim
 1. 6. Analkali-soluble nitrone compound according to claim 1, wherein saidnitrone compound is of the formula ##STR14## wherein R³, R⁴, R⁵, R⁶, R⁷,R⁸, R⁹ have, respectively, the same meanings as defined in claim
 1. 7.An alkali-soluble nitrone compound according to claim 1, wherein saidnitrone compound is of the formula ##STR15## wherein R¹, R², R³ , R⁴,R⁵, R⁶, R⁷, R⁸ R¹⁰ and R¹¹ have, respectively, the same meanings asdefined in claim 1 and m is 1 or
 2. 8. An alkali-soluble nitronecompound according to claim 1, wherein said nitrone compound is of theformula ##STR16## wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, and R⁹ have,respectively, the same meanings as defined in claim 1 and m is 1 or 2.9. A contrast enhanced material which comprises an alkali-solublenitrone compound of the following formula ##STR17## wherein R¹, R² andR³ may be the same or different and represent an alkyl group, an arylgroup or a hydrogen atom, R⁴ to R⁸ may be the same or different andrepresent an alkyl group, a hydrogen atom or a carboxyl group providedthat at least one of R⁴ to R⁸ is a carboxyl group, X represents analkoxy group of the formula, R⁹ O--, wherein R⁹ represents an alkylgroup, a dialkylamino group of the formula, R¹⁰ R¹¹ N--, wherein R¹⁰ andR¹¹ may be the same or different and represent an alkyl group, or ahydrogen atom, and n is a value of 0, 1 or
 2. 10. A contrast enhancedmaterial according to claim 9, which comprises from 0 to 100 parts byweight of water, from 0 to 100 parts by weight of an organic solvent,from 1 to 30 parts by weight of a water-soluble polymer binder, from 1to 30 parts by weight of said nitrone compound, from 1 to 30 parts byweight of an organic base, and from 0 to 2 parts by weight of a surfaceactive agent.
 11. A contrast enhanced material according to claim 10,which comprises from 50 to 100 parts by weight of water, from 0 to 50parts by weight of an organic solvent, from 1 to 15 parts by weight of awater-soluble polymer binder, from 1 to 15 parts by weight of saidnitrone compound, from 1 to 15 parts by weight of an organic base, andfrom 0 to 1 part by weight of a surface active agent.
 12. A contrastenhanced material according to claim 9, wherein R¹, R² and R³ areindependently an alkyl group having from 1 to 8 carbon atoms.
 13. Acontrast enhanced material according to claim 9, wherein R¹, R² and R³are independently an aryl group having from 6 to 15 atoms.
 14. Acontrast enhanced material according to claim 9, wherein n is 0 or 1.15. A contrast enhanced material according to claim 9, wherein saidmaterial is of the formula ##STR18## wherein R³, R⁴, R⁵, R⁶, R⁷, R⁸, R¹⁰and R¹¹ have, respectively the same meanings as defined in claim
 9. 16.A contrast enhanced material according to claim 9, wherein said materialis of the formula ##STR19## wherein R³, R⁴, R⁵, R⁶, R⁷, R⁸, and R⁹ have,respectively the same meanings as defined in claim
 9. 17. A contrastenhanced material according to claim 9, wherein said material is of theformula ##STR20## wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R¹⁰ and R¹¹have, respectively the same meanings as defined in claim 9 and m is 1 or2.
 18. A contrast enhanced material according to claim 9, wherein saidmaterial is of the formula ##STR21## wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷,R⁸, and R⁹ have, respectively the same meanings as defined in claim 9.19. An alkali-soluble mixture nitrone compound according to claim 1,wherein R⁴ to R⁵ are each independently an alkyl group having 1-8 carbonatoms, a hydrogen atom or a carboxyl group, provided that at least oneof R⁴ to R⁸ is a carboxy group, and R⁹, R¹⁰ and R¹¹ are eachindependently an alkyl group having 1-8 carbon atoms, or a hydrogenatom.